1. Field of the Invention
The present invention relates to design, synthesis and application of oligonucleotide analogues which are useful as therapeutics, diagnostics as well as research reagents. Oligonucleotide analogues are provided that incorporate an analogue of cytosine, 5-aza-cytosine, in the oligonucleotide sequence, e.g., in the form of 5-aza-2′-deoxycytidine or 5-aza-cytidine. Such analogues can be used for modulation of DNA methylation, especially for effective inhibition of methylation of cytosine at the C-5 position by more specifically targeting the CpG islets of the human genome. Methods for synthesizing these oligonucleotide analogues and for modulating C-5 cytosine methylation are provided. In particular, phosphoramidite building blocks and oligonucleotides containing decitabine (5-aza-2′-deoxycytidine; D), DpG-rich (Decitabine-phosphodiester linkage-Guanosine) islets and derivatives, are provided. Also provided are methods for preparing, formulating and administering these compounds or compositions as therapeutics to a host in need thereof.
2. Description of Related Art
Decitabine is currently being developed as a new pharmaceutical for the treatment of chronic myelogenous leukemia (CML), myelodysplastic syndrome (MDS), non-small cell lung (NSCL) cancer, sickle-cell anemia, and acute myelogenous leukemia (AML). Two isomeric forms of decitabine can be distinguished. The β-anomer is the active form, which is shown in FIG. 1. Decitabine possesses multiple pharmacological characteristics. At the molecular level, it is incorporated into DNA during the S phase of cell cycle. At the cellular level, decitabine can induce cell differentiation and exert hematological toxicity. Despite having a short half-life in vivo, decitabine has an excellent tissue distribution.
One of the functions of decitabine is its ability to specifically and potently inhibit DNA methylation. DNA methylation is an epigenetic effect common to many systems. This modification involves the covalent modification of cytosine at the C-5 position (1a″). Methylation patterns are stably maintained at CpG dinucleotides by a family of DNA methyltransferases that recognize hemimethylated DNA after DNA replication. Inside the cell, decitabine is first converted into its active form, the phosphorylated 5-aza-deoxycytidine, by deoxycytidine kinase, which is primarily synthesized during the S phase of the cell cycle. The affinity of decitabine for the catalytical site of deoxycytidine kinase is similar to the natural substrate, deoxycytidine (Momparler et al. 1985 Mol. Pharmacol. 30:287-299). After conversion to its triphosphate form by deoxycytidine kinase, decitabine is incorporated into replicating DNA at a rate similar to that of the natural substrate, dCTP (Bouchard and Momparler 1983 Mol. Pharmacol. 24:109-114).
CpG-rich sequences of housekeeping genes are generally protected from methylation in normal cells. In cancerous cells, aberrant hypermethylation in promoter region CpG-islands of tumor suppressor genes is one of the most common events associated with progression of the tumorigenic phenotype. Each class of differentiated cells has its own distinct methylation pattern. Incorporation of decitabine into the DNA strand has a hypomethylation effect. After chromosomal duplication, in order to conserve this pattern of methylation, the 5-methylcytosine on the parental strand serves to direct methylation on the complementary daughter DNA strand. Substituting the carbon at the C-5 position of the cytosine for nitrogen interferes with this normal process of DNA methylation. The replacement of cytosine with decitabine at a specific site of methylation produces an irreversible inactivation of DNA methyltransferases. Decitabine behaves faithfully as a cytosine residue until DNA methyltransferase enzymes attempt to transfer a methyl group to the hemimethylated DNA strands of the daughter cells. At this step the DNA methyltransferase enzyme is covalently trapped by decitabine in the DNA and cannot further silence (methylate) additional cytosine residues (Juttermann et al. 1994 Proc. Natl. Acad. Sci. USA 91:11797-11801). This unique mechanism of action of decitabine allows genes silenced (that were once methylated) from previous rounds of cell division to be re-expressed. The active trap is present in the hemimethylated DNA up to 48 hours after decitabine treatment. After further DNA synthesis and cell cycle division, progeny strands from the hemimethylated DNA result in DNA strands that are completely unmethylated at these sites (Jones P. 2001 Nature 409: 141, 143-4). By specifically inhibiting DNA methyltransferases, the enzyme required for methylation, aberrant methylation of the tumor suppressor genes could be reversed.
Despite its proven antileukemic effects in CML, MDS, and AML, the potential application of decitabine has been hampered by delayed and prolonged myelosuppression. Lower doses of decitabine, given over a longer period of time, have minimized myelosuppression to manageable levels without compromising its ability to suppress cancer via its hypomethylation effect. At higher doses, the associated toxicity was prohibitive. However, treatment of hematologic and solid tumors at maximally tolerated doses of decitabine has been ineffective. The cause of myelosuppression is not clear. It is plausible that since decitabine is randomly and extensively incorporated into the DNA of S phase cells, including bone marrow cells that are involved in normal hematopoiesis, the severe DNA damage due to the instability of decitabine leads to necrosis. Since incorporation of decitabine is not restricted to only the CpG-rich sequences, the DNA can break, due to the instability of decitabine, and require repair at numerous sites outside of the CpG islands.
Decitabine and azacitidine are unstable in aqueous media and undergo hydrolytic degradation. In acidic medium, decitabine is hydrolyzed at room temperature to 5-azacytosine and 2-deoxyribose. In neutral medium at room temperature, the opening of the triazine ring takes place at the 6-position to form the transient intermediate formyl derivative, which further degrades to the amidino-urea derivative and formic acid (Piskala, A.; Synackova, M.; Tomankova, H.; Fiedler, P.; Zizkowsky, V. Nucleic Acids Res. 1978, 4, s109-s-113). This hydrolysis at the 6-position occurs in acidic and basic aqueous media at even faster rates.
In view of the chemical instability and toxicities associated with decitabine, there exists a need to develop not only more stable derivatives of decitabine but superior hypomethylating agents, where incorporation is localized to the CpG islands as much as possible or hypomethylation is achieved without significantly affecting the integrity of the DNA.